On a day when Democrats buried their longest-serving colleague, Senator Robert C. Byrd of West Virginia, who died last week at the age of 92, hoax news releases announced the death of Mr. Leahy, following similar announcements about Mrs. Feinstein and Mr. Lautenberg.

The hoax messages were sent to various news organizations, including The New York Times, which received the notice about Mr. Leahy, and said that the senators had died of liver cancer.

A close look at the detailed header of the message makes clear that it did not originate from the Senate computer system but rather from an outside domain, 000.webhost.com — a sign that government computers were not hacked.

The Senate sergeant at arms and the Capitol Police are investigating the messages, officials said.

The message to The Times about Mr. Leahy was received at 2:35 p.m. on Tuesday. An aide to Mrs. Feinstein said the e-mail message about her went out sometime after midnight Monday, prompting a call from a reporter at 3 a.m.

Mr. Leahy, who marched in Fourth of July parades over the weekend, was home in Vermont and joked about the premature reports of his demise on Vermont Public Radio, but he also said he wanted the e-mail investigated.

The hoax was perhaps cruelest in the case of Mr. Lautenberg, who recently completed treatment for lymphoma of the stomach. Last Monday, he announced he was cancer free — on the same day that Mr. Byrd’s death made Mr. Lautenberg, at 86, the oldest sitting senator.

In each case, the fake announcements included phone numbers associated with the senator’s offices and the names of staff members. But in at least two of the cases, the staff members named were not the aides who normally send out news releases.

A spokesman for Senator Harry Reid, Democrat of Nevada and the majority leader, said other senators, as well as the Capitol press galleries, would be alerted about the hoax messages.

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